1. Field of the Invention
The disclosure taught herein relates generally to a system and method for assembly and deployment of an extendable draft platform for deepwater offshore exploration and production of oil and gas, and more specially, relates a connection and locking system between a deck and columns.
2. Description of the Related Art
The development of deep water offshore oil and gas fields, such as are found in the Gulf of Mexico and the North Sea, present substantial challenges to the industry. Early production schedule requirements favor onshore or near shore integration and commissioning and a year-round deployment capability. Moreover, the ability to use so-called “dry trees” and steel catenary risers (“SCRs”) requires that the motion of the deployed structures be relatively small, even in rough seas.
One type of offshore platform that has met with commercial success in deep water applications is the semi-submersible platform. Conventional semi-submersible platforms, however, are subject to motions that make it difficult, or even impossible, to support the various types of risers that are employed in such platforms. Deep draft semi-submersible platforms have been proposed that would exhibit superior motion characteristics. One type of deep draft semi-submersible platform is known as the extendable draft platform, or “EDP.” The typical EDP comprises a buoyant equipment deck having a plurality of openings (“leg wells”) through the deck. The deck may conveniently be rectangular or triangular, with a leg well at each corner or apex, although other configurations may be used. A buoyancy column or “leg” is installed in each of the leg wells that can be ballasted, for example, with seawater. The columns are initially installed in a raised position, and then lowered to a submerged position when the EDP has been moved to a deeper water site. Each column is divided by longitudinal internal bulkheads and horizontal flats (decks) into a plurality of compartments, the compartments including generally known system for introducing water into them for ballasting purposes when the columns are lowered to their submerged positions. Further, a heave plate pontoon assembly is generally attached to the bottom of the columns that helps to stabilize the EDP against the heave action of waves and swells. Examples of prior art EDPs are disclosed in U.S. Pat. No. 6,718,901 and U.S. Pat. No. 7,037,044.
An attachment or connection mechanism between the deck and columns needs to allow the downward movement of the columns relative to the deck as the columns are lowered to their deployed, submerged position prior to the attachment or connection of the columns to the deck. The attachment or connection mechanism generally should not interfere with the column lowering process. However, once the columns are lowered to their submerged position, the tops of the columns need to support the deck, so that as the columns are raised after connection to the deck, the deck is supported above the water surface. It is important that the attachment or connection of the columns to the deck be secure enough to withstand the strong forces that sea currents, rough waves, and wind often apply to the EDP.
Prior art has provided various solutions to connecting the columns to the deck. For example, in U.S. Pat. No. 6,718,901, the legs in a deployed position support the deck by moving a pawl from the legs to a position in compression or shear under the deck. Chains using winches pull the legs to the deck. Because the pawl only stops the legs from moving upward relative to the deck, the system is dependent on the chains to hold the deck and legs together, so that the legs do not move downward relative to the deck. The deck can be more permanently connected to the legs, such as by welding after deployment, which would need cutting off to be able to retract the legs at a later time when the EDP is moved.
U.S. Pat. No. 7,037,044 has several embodiments that generally depend on a locking pin engaging an aperture; or a column that can be temporarily positioned and welded to the deck. The locking pins can bend under the load or under a shear stress from ballasting or de-ballasting the column to support the deck above the water surface and cause difficulties in removal; the weldments need cutting off to be able to disconnect the column from the deck when the EDP is moved; and the tapered load on the actuation means can compromise the ability to maintain the tapered shear key in position so that an unintended upward movement of the column relative to the deck can occur. Other systems have used a rack-and-pinion lowering systems which can be locked in various ways.
While prior art approaches have provided satisfactory results, further improvements in the connection/attachment mechanisms have been sought, so as better to achieve the intended results.
There remains a need for a different system and method for allowing movement and locking and unlocking of the columns with the deck in an EDP or other offshore platform.